Electronic systems and circuits have made a significant contribution towards the advancement of modern society and are utilized in a number of applications to achieve advantageous results. Numerous electronic technologies such as digital computers, calculators, audio devices, video equipment, and telephone systems include processors that have facilitated increased productivity and reduced costs in analyzing and communicating data, ideas and trends in most areas of business, science, education and entertainment. Frequently, electronic systems designed to provide these results include integrated circuits (ICs) on chip wafers. Usually, the wafers are produced by processes that include a chemical mechanical polishing (CMP) step. Typical CMP processes include the application of a chemical slurry that assists a chemical/mechanical abrasion step that polishes and planarizes the wafer. To be effective and operate properly, most CMP processes require an efficient distribution of the chemical slurry.
The starting material for typical ICs is very high purity silicon. The pure silicon material is grown as a single crystal that takes the shape of a solid cylinder. This crystal is then sawed (like a loaf of bread) to produce wafers upon which electronic components are then constructed by adding multiple layers to the wafer through a process of lithography (e.g., photolithography, X-ray lithography, etc.). Typically, lithography is utilized to form electronic components comprising regions of different electrical characteristics added to the wafer layers. Complex ICs can often have many different built up layers, with each layer being stacked on top of the previous layer and comprising multiple components with a variety of interconnections. The resulting surface topography of these complex IC's are bumpy (often resemble familiar rough terrestrial "mountain ranges" with many rises or "hills" and dips or "valleys") after the IC components are built up in layers.
Lithographic techniques are usually able to reproduce very fine surface geometry and greater advantages and usefulness are realized in applications in which more components (resistors, diodes, transistors, etc.) are integrated into an underlying chip or IC. The primary manner of incorporating more components in a chip is to make each component smaller. In a photolithographic process, limitations on the depth of focus impact the projection of increasingly finer images onto the surface of the photosensitive layer. Depth of focus problems are exacerbated by rough topographies (e.g., the bumpy rises and dips causes by layers produced during lithographic processes). The "bumpy" topography of complex ICs, the "hills" and "valleys," exaggerate the effects of narrowing limits on the depth of focus which in turn limits the number of components that are incorporated on a chip. Thus, in order to focus desirable mask images defining sub-micron geometries onto each of the intermediate photosensitive layers in a manner that achieves the greatest number of components on a single wafer, a precisely flat surface is desired. The precisely flat or fully planarized surface facilitates extremely small depths of focus operations, and in turn, facilitates the definition and subsequent fabrication of extremely small components.
Chemical-mechanical polishing (CMP) is the preferred method of obtaining full planarization of a wafer layer. It usually involves removing a sacrificial portion of material by rubbing a polishing pad covered with a polishing slurry on the surface of the wafer. CMP flattens out height differences on the surface of the wafer, since high areas of topography (hills) are removed faster than areas of low topography (valleys). Most CMP techniques have the rare capability of smoothing out topography over millimeter scale planarization distances leading to maximum angles of much less than one degree after polishing.
As described above, most CMP processes use an abrasive slurry dispensed on a polishing pad to aid in the smooth and predictable planarization of a wafer. The planarizing attributes of the slurry are typically comprised of an abrasive frictional component and a chemical reaction component. The abrasive frictional component is due to abrasive particles suspended in the slurry. The abrasive particles add to the abrasive characteristics of the polishing pad as it exerts frictional contact with the surface of the wafer. The chemical reaction component is attributable to polishing agents which chemically interact with the material of the wafer layer. The polishing agents soften and/or dissolve the surface of the wafer layer to be polished by chemically reacting with it. Together the abrasive frictional component and a chemical reaction component assist a polishing pad to remove material from the surface of the wafer.
The slurry utilized in CMP processes is typically a mixture of de-ionized water, abrasives and polishing agents. The constituents of the slurry are precisely determined and controlled in order to effect optimized CMP planarization. Differing slurries are used for differing layers of the semiconductor wafer, with each slurry having specific removal characteristics for each type of layer. As such, slurries used in extremely precise sub-micron processes (e.g., tungsten damascene planarization) can be very expensive and often represent the most expensive consumable used in the CMP process.
The friction caused by the contact between the rotating polishing pad and the rotating wafer, in conjunction with the abrasive and chemical characteristics of the slurry, combine to remove a top portion of the wafer layer and planarize or polish the wafer at some nominal rate. This rate is referred to as the removal rate. A constant and predictable removal rate is important to the uniformity and performance of the wafer fabrication process. The removal rate should be expedient, yet yield precisely planarized wafers, free from a rough surface topography. If the removal rate is too slow, the number of planarized wafers produced in a given period of time decreases, degrading wafer through-put of the fabrication process. If the removal rate is too fast, the CMP planarization process will not be easy to control and a small variation can impact uniformity and degrade the yield of the fabrication process.
The slurry is usually applied to the polishing pad and transported to the surface of the wafer by the pad. A polishing pad usually has a roughened surface comprising a number of very small pits and gouges that function to efficiently transport slurry to the wafer surface being polished. The efficient transport of slurry produces a fast and consistent removal rate. The polishing pad texture is usually comprised of both the inherently rough surface of the material from which the polishing pad is made and predefined pits and grooves that are manufactured into the surface of the polishing pad. The pits and grooves act as pockets that collect slurry for transportation to and from the wafer. To aid in maintaining the surface quality of a polishing pad, CMP machines typically include a conditioner which is used to roughen the surface of the polishing pad. Without conditioning, the surface of the polishing pad is smoothed during the polishing process and removal rates decrease dramatically. As slurry is "consumed" in the polishing process, the transport of fresh slurry to the surface of the wafer and the removal of polishing by-products away from the surface of the wafer becomes very important in maintaining the removal rate.
The manner in which the slurry is distributed to the polishing pad significantly impacts the effectiveness of the abrasive and chemical characteristics of the slurry in aiding the polishing, which in turn impacts the removal rates. It is important to evenly distribute the slurry over the surface of the pad and wafer so that the removal of the wafer layer is even. If a portion of the wafer is exposed to contact with an excessive amount of slurry it usually is removed at a faster rate and portions that are not exposed to enough slurry is usually removed at a slower rate, creating a rough topography instead of a planarized one. For the same reason, it is also preferable to avoid agglomeration of the slurry particles. Agglomeration of slurry particles is a common problem with typical CMP slurries.
What is required is a system and method that facilitates an efficient application of a slurry in an effective manner to the surface of a polishing pad. The system and method should support an even and disperse distribution of slurry particles while reducing slurry consumption. It should also aid conditioning processes to prepare a pad for continued use.